This invention relates to permeate carrier fabrics for membrane filters, such as reverse osmosis filters, ultrafiltration filters and other types of membrane filters. Such filters are now in use in many applications for high-efficiency liquid filtration. Such membrane filters appear in spiral wound elements and are used with reverse osmosis filtration systems, microfiltration systems, nanofiltration systems, as well as ultrafiltration systems.
As stated above, such membrane filters generally comprise spiral wound elements made of three components, the membrane, the permeate carrier fabric, and the feed spacer. The membrane is the part of the element where the separation occurs and can be either a reverse osmosis, nanofiltration, ultrafiltration, or microfiltration type membrane. The feed spacer, on the other hand, separates two adjacent membrane faces and acts as a spacer and also a turbulence promoter. The permeate carrier fabric is placed between layers of the membrane and acts as a pipe to allow the permeate to flow between the adjacent membranes and exit from the element. A composite is made which consists of a first membrane layer, an intermediate permeate carrier fabric, and a second membrane layer. These three components are glued or sonically welded together on three sides, and numerous layers of these three-part composites are each glued around a perforated filter core. The spacer fabric is used between each layer of composite. The layers are rolled around the core to a certain diameter based on the size/diameter of the filter. This element is then placed in a cylindrical container.
As stated above, the purpose of the permeate carrier fabric is to provide direction for and channel the flow of the liquid. It is therefore important that the yarns in the fabric be sufficiently stiff or firm to prevent collapse.
The permeate carrier fabric is generally a knitted polyester tricot fabric. This fabric, as stated above, is placed between permeable membranes. Tricot knitted fabric has been found to be a particular desirable structure for supporting the membrane material due to the porous knitted structure and raised rows of stitches which define between them long, continuous channels akin to corrugation through which the liquid being filtered flows. However, other types of fabric may also be used as the permeate carrier, including other types of knitted fabric or even, woven fabric. The permeate carrier fabric should have a low pressure drop (high permeability) for the permeate flow while being able to withstand the high pressures exerted by the liquid being filtered without compaction.
In the past, there have generally been two types of permeate carrier fabrics. One type are knit fabrics of multifilament polyester yarns, which yarns are then coated with a resin to add firmness to the fabric. The other type of permeate fabric are fabrics made with bi-component yarns having a regular polyester core and a low melt polyester sheath. This sheath part of the yarn is melted during finishing to give the yarn its necessary firmness. Resin applied to add firmness to the resin coated yarns also tends to partially block the fabric channels thereby restricting part of the permeate flow. When the yarns in the bi-component fabrics are melted, again, the same result occurs, the channels are again partially blocked restricting the permeate flow.